Please accept this free advice from a packrafting novice who began when he lived in Texas. These are numbered in accordance with your questions.
1. Go bigger rather than smaller, and don't get a Scout if your goal includes the West's big rivers. The Scout is more of a stillwater (lake) boat or river tender. Your extended legs are part of the boat's frame, and if you're going to be paddling all day you want to be reasonably comfortable, not scrunched up.
2. Get a spray deck if any river running is on your agenda. Keeping water out of the boat reduces the number of times you'll have to eddy out to dump the water.
3 and 4. Lakes are a good place to start, but if you select a mountain lake don't go too far from shore as high winds can make things daunting. Very true of Jenny Lake. An easy and very scenic trip is the Lewis Channel to Shoshone Lake in Yellowstone National Park. If you select this don't forget to register your raft and obtain the necessary camping permits. If you can backpack in with your raft, consider also the lakes on the Lake Plateau in the Absaroka-Beartooth Wilderness in Montana, another scenic wonder.
5.This summer the Northern Rockies will likely have high water into mid-August at least. Very high snowfall this winter.
One other suggestion - take one of BPL's packrafting courses. You'll learn a lot and have a great time.